Synthetic aperture radars (SARs) are used to obtain high-resolution images of the ground in all weather conditions from a moving carrier. They illuminate the ground in a direction lateral to the route of the carrier, by a coherently repeated recurrent electromagnetic waveform, receive the sum of the echoes sent back by the ground and analyze this sum of echoes received throughout the route of the carrier for the deduction therefrom of the reflection coefficients of the different points of the zone examined in order to form a radar image. The phase coherence from one recurrence to another enables analysis of the Doppler history of the different targets and therefore makes it possible to separate them in azimuth, i.e. separate them along the direction of motion of the carrier identified on an axis known as an azimuth axis. The resolution in the direction perpendicular to that of the movement of the carrier identified by an axis called a range axis is done as a function of the echo return time. It is improved conventionally by the pulse compression technique.
The data elements delivered at reception by an SAR type radar are constituted by a sampling of the unprocessed video signal received at a double rate: a fast rate corresponding to the sequence of range gates that subdivide the listening time of the radar after each transmission of a waveform and a slow rate corresponding to the succession of recurrences of the transmitted waveform. The result thereof is that, for an image processing operation, there is available a data table with two dimensions: a range dimension along which the samples of the unprocessed video signal received in response to the transmission of a waveform are aligned, these samples being taken at the fast sampling rate, and an azimuth dimension along which there are arranged the successive sequences of samples of the unprocessed video signal received for the various recurrences of the transmitted waveform.
The image processing operation consists of the extraction of an image from a 2D table such as this that brings together the unprocessed video samples received by the radar during the illumination of a zone. This extraction is done by separating the contributions, to the total echo, of each point of the zone in taking account of the specific progress of the range and azimuth coordinates of the point considered during the illumination of this point by the radar moving above the zone. The range coordinate progresses as follows: the radar approaches the point until it passes through the perpendicular to this point and moves away from this point. The phenomenon is known as the range migration phenomenon. The progress of the azimuth coordinate consists of an approach at decreasing speed implying a Doppler effect at decreasing positive frequency followed by a moving away at increasing speed implying a Doppler effect at increasing negative frequency with cancellation of the Doppler effect when the radar passes through the perpendicular to the point. This phenomenon is known as the Doppler history.
If we set aside the pulse compression, the image processing consists of the following steps for each point of the zone illuminated by an SAR type radar: selecting the azimuth domain in the table of unprocessed video samples corresponding to the period of illumination of the point considered by the radar; then, in this azimuth domain, identifying the band of samples to which the point considered has contributed by its echo, in taking account for this purpose of the range migration; finally, filtering the collection of samples of this band to take account of the Doppler history of the point considered and extract its complex reflection coefficient therefrom. These operations which are necessary for each point of an imaged zone are done so as to limit, as far as possible, the quantity of calculations performed.
A theoretical analysis of the nature of the operations, including a possible preliminary pulse compression operation, that have to be conducted on the 2D range and azimuth table of the unprocessed video reception samples delivered by an SAR type radar in order to obtain an image, shows that these operations can be interpreted as a 2D filtering of the table in the range and azimuth dimensions. This filtering is done by means of a so-called image focusing filter whose pulse response h(.tau.,t,.tau..sub.i) to two temporal variables, .tau. range and t azimuth, is not stationary in range. This justifies the presence of a second range parameter .tau..sub.i in its expression. From this, there follows a standard method of processing the table of unprocessed video reception samples delivered by an SAR type radar. This standard method consists in:
determining the pulse response h(.tau.,t,.tau..sub.i) to two temporal variables, namely .tau. range and t azimuth, locally valid in the range bands .tau..sub.i where this response is assumed to remain stationary, that define the image focusing 2D filter carrying out the following at the same time: the pulse compression, the correction of range migration and the extraction of the frequency components corresponding to the Doppler history. This determining of the pulse response is done on the basis of the properties of the waveform transmitted and the geometrical parameters of the image being taken, PA1 convoluting this image focusing filter pulse response with the table of unprocessed video signal samples received, subdivided beforehand into range bands through direct and reverse 2D Fourier transform operations, to return to the spaces of the range and azimuth frequencies and avoid integration computations inherent in a convolution, and PA1 combining the image bands obtained to reconstitute a full image. PA1 the passage of the data elements from the reception data table into a dispersive delay line with a pulse response h.sub.l (t) such that its instantaneous frequency is a linear function of the time; ##EQU1## with a linear modulation slope having a frequency K chosen to be equal to: ##EQU2## T.sub.a being the useful duration of demodulation or duration of a common temporal support chosen for the demodulation of the echo signals coming from all the targets of the useful swath, this useful duration T.sub.a being smaller than or equal to the duration T.sub.d of the demodulation ramp and greater than or equal to the period of time beginning before the start of reception of an echo sent by a target placed in distance at the far end of the useful swath and ending after the end of reception of an echo sent by a target placed in distance at the near end of the useful swath while at the same time being centered in distance on the middle of the useful swath, PA1 the selection, from among the data elements of the 2D table, of reception data elements, consisting of the withdrawal of the reception data elements arriving along the range axis outside the useful duration chosen T.sub.a and their replacement by zero values, PA1 the replacement of the data elements of the table of reception data, considered in the range dimension, by their Fourier transform, which is one-dimensional in range, for the obtaining, after demodulation and passage through the dispersive line, of a type of pulse compression to which there is assigned a parasitic phase term of pulse compression, PA1 the rearrangement of the data elements of the table in the range dimension in order to have available data corresponding to an order of moving away that increases in range, PA1 the subdivision of the table into overlapping bands, parallel to the azimuth axis, so as to have bands corresponding to narrow zones of range .tau..sub.i where it is possible, as a function of the geometrical parameters of the image taken, to locally determine a image focusing 2D filter having a pulse response h(.tau.t,t,.tau..sub.i) with two temporal variables, namely .tau. range and t azimuth, and a function of correction of the parasitic phase due to the pulse compression, that are stationary in the range band .tau..sub.i, PA1 the filtering of the table bands by the image focusing 2D filter whose pulse response has been modified by the parasitic phase correction function, PA1 the juxtaposition of the table bands resulting from the filtering to obtain a table of complex reflection coefficients of the points of the illuminated region of ground, and PA1 the construction of an image of the illuminated region from the moduli of the complex reflection coefficients represented in the table obtained in the previous step.
It is shown that a certain economy of computation can be achieved by performing the pulse compression operations prior to the image processing operations.
The pulse compression which makes it possible to improve the range resolution can be done on several waveforms emitted, and especially on a waveform constituted by a coherently repeated linear frequency-modulated pulse. In theory, the pulse compression consists in filtering the signal received with a filter matched to the waveform transmitted or again in correlating the received signal with the transmitted waveform. This operation is difficult to achieve, and is often costly in terms of computation. This is why it is often sought to replace it with simpler processing operations giving similar results. One example thereof is provided by a type of radar known as the Deramp radar which transmits coherently repeated linear frequency-modulated microwave pulses and carries out a sort of pulse compression in reception, not by matched filtering but by a demodulation of the signal received by the transmitted pulses and a frequency analysis of the demodulated signal received. For details of the Deramp radar, reference may be made to: [1] J. P. HARDANGE, P. LACOMME and J. C. MARCHAIS, "Radars aeroportes et spatiaux" (Airborne and space radars), Masson 1995, pp. 168-170.
In a Deramp radar, advantageous use is made of the fact that with a linear frequency-modulated pulse, there is a relationship of proportionality between the time that elapses and the instantaneous frequency of the pulse. Through this relationship, the mixing of two pulses that are staggered in time such as a transmitted pulse and the echo pulse that is returned by a point target gives a signal whose frequency is constant with respect to time and depends on the relative delay between the two pulses. Thus, the correlation between the transmitted and received waveforms that results in the pulse compression can be replaced by a simple demodulation of the signal received by the transmitted signal followed by a Fourier transform with minor differences in the result.
The operational advantage of the processing done in a Deramp radar with a view to pulse compression is that it enables very high range resolution with a narrow instantaneous band receiver. In the case of use in imaging, this is done to the detriment of the width of the processed swath, i.e. the useful duration of reception between each waveform transmitted for the useful instantaneous band in the receiver is proportional to the duration of the swath processed. This mode of pulse compression is therefore particularly suited to very high resolution radar imaging systems on regions that are not extensive in distance.
The use of a Deramp type radar as a synthetic aperture radar to carry out radar imaging raises difficulties owing to the particular properties of its demodulated reception signal. Indeed, this signal has the property, in relation to the target echo signal from which it originates, of having a delay that is variable as a function of the position in range of the target in the swath, namely in the zone illuminated by the transmitted pulse and also of having a variable duration, smaller than that of the target echo signal from which it originates, depending also on the position in range of the target in the swath. Its delay, which is variable with respect to the echo signal from which it originates, generates a parasitic phase that disturbs the subsequent operation of image construction processing and adversely affects the sharpness of the image while its variable duration gives the Deramp type radar a resolution that is variable in range and also affects the sharpness of the image.
The present invention is aimed at resolving these difficulties in order to obtain a radar image that is as sharp as possible.